A method for measuring effective crack tip fracture toughness in bridging materials

A method for measuring the effective crack tip fracture toughness, (Ktip)c, for bridging materials such as polycrystalline ceramics or fiber-reinforced ceramics is proposed in this paper. The value of (Ktip)c approximately coincides with the apparent fracture toughness, Kc, when using a bending specimen with an adequately deep crack. The stress intensity factor for the bending specimen with a deep crack increases very rapidly with crack extension. Therefore, the bending specimen with a deep crack fractures without stable crack propagation which causes the R-curve behavior and the deference between Kc and (Ktip)c. The volue of (Ktip)c for polycrystalline alumina is evaluated according to the present proposed method. The measured (Ktip)c is almost identical with that determined by numerical analysis.